The hippocampus contains diverse types of GABAergic inhibitory neurons. Previous work suggests that different subtypes of these inhibitory neurons have distinct functions, but the rules for their regulation of the hippocampal circuit remain to be determined. We hypothesize that functional differences between inhibitory neurons result from distinct circuit connections of different types of inhibitory neurons. The goal of the proposed studies is to map local and long-range direct synaptic connections to major inhibitory neuronal types in CA1 of the mouse hippocampus. We hypothesize that specific types of inhibitory neurons selectively receive local and distant excitatory synaptic inputs from different brain regions, and that each of these inputs to specific inhibitory neurons differentially contributes to their inhibitory regulation of hippocampal target neurons. Specifically, (1) we aim to identify local excitatory connections to specific types of inhibitory hippocampal neurons by laser scanning photostimulation (LSPS). We have combined LSPS with whole-cell recordings from inhibitory neurons in living brain slices to map intrahippocampal sources of excitatory input to the most numerous inhibitory cell types including parvalbumin-expressing (PV+) basket cells, cholecystokinin- expressing (CCK+) basket cells, axo-axonic cells, and somatostatin-expressing (SOM+) oriens-lacunosum moleculare (O-LM) cells. We will test the hypothesis that axo-axonic cells, PV+ and CCK+ basket cells receive differential strength of excitatory input from CA3 vs. CA1 to support their feedforward and feedback inhibition of CA1 network activity. We will also test the hypothesis that O-LM cells only receive excitation from excitatory pyramidal cells in CA1 and strictly perform local feedback inhibition. (2) We aim to identify long-range synaptic connections to selected groups of inhibitory hippocampal neurons with a novel rabies-based tracing system and optogenetic stimulation. We will use knock-in mouse lines that express Cre (Cre recombinase) in selected groups of inhibitory neurons (e.g., PV-Cre, SOM-Cre or CCK-Cre) to limit rabies infection and monosynaptic retrograde tracing to each selected cell group in the intact brain. The rabies tracing will be followed by channelrhodopsin-assisted circuit mapping to functionally characterize the specificity of distant connections to identified cell type within each targeted Cre-expressing cell group. We will test the hypotheses that PV+ inhibitory cells, but not SOM+ inhibitory cells, receive strong distant connections from entorhinal cortex and the medial septum, and that the CCK+ cell group has strong direct synaptic connections with the amygdala. These studies should establish the operational rules of the major classes of inhibitory neurons within the hippocampal circuit. PUBLIC HEALTH RELEVANCE: The proposed studies should increase our mechanistic understanding of inhibitory neuronal circuit organization in the hippocampus. This will guide future studies to assess and treat circuits in the brains that are altered following disease or injury. The results of this research also will enable better therapeutic targeting of neuronal components disrupted by disease in hippocampal circuits that contribute to epilepsy and learning and memory disorders.